Combiner filters, usually of cavity or dielectric type, are used to connect output signals from several power amplifiers and with different frequencies to a common antenna. The combiner filters are tuned to the frequencies that are used in the respective power amplifiers. Since the transfer functions of the combiner filters are not ideally rectangular unacceptable phase and amplitude distortion of the signal that passes through a combiner filter can arise. As an example the frequency dependent group delay in the combiner filter induces a phase error. Furthermore, the non-constant amplitude response within the modulation band width generates an amplitude pulsation. This means in particular that single pole filters cannot be used if a very large channel distance, that allows wide filters, is not used.